Patients treated with chemotherapy complain of decreased cognitive functioning before and during chemotherapy, as well as for some years after the end of therapy. Objective measures of cognitive functioning confirm that a large percentage of patients receiving chemotherapy show at least acute decrements in multiple domains of cognitive test performance. This phenomenon has been termed "chemobrain." Understanding the cause of the cognitive deficits is critical as these patients require increased care, experience impaired decision making ability, experience decreased quality of life and express concern about their ability to maintain employment. Fatigue and problems sleeping are also major complaints in this population, particularly in patients with breast cancer. Yet whether chemobrain is related just to the chemotherapy, or might be secondary to fatigue and/or sleep and circadian rhythm problems has not yet been explored. Clarification of the role of sleep disturbances, hormonal changes, anxiety and depression, and other proximal causes of cognitive impairment in chemotherapy patients may be helpful in identifying potential targets for secondary intervention. The specific aims are: to examine how much cognitive impairment occurs during chemotherapy and one year after the start of chemotherapy compared to before the start of treatment; to examine the relationship between cognitive impairment and fatigue; to examine the relationship between cognitive impairment and sleep and circadian rhythms; and to examine the relationship between inflammatory markers, cognitive functioning and sleep. Over 5 years, 80 women with breast cancer and 80 controls (women with no history of any cancer) will be studied. Women will have their sleep recorded for three days with actigraphy, a complete neuropsychological test battery, have blood drawn for inflammatory markers and complete questionnaires on fatigue, sleep, menopausal status, quality of life, depression, and anxiety. These procedures will be repeated before the start of chemotherapy, during week 2 of cycle 4 of chemotherapy and one year after chemotherapy. Control women will be tested at the same time points. Each woman will also have a complete overnight sleep recording. The reported phenomena of chemobrain will be examined to see if it is related to the complaint of fatigue or sleep disruptions, both of which are known to contribute to decreases in cognitive functioning.